On Mon, Jul 10, 2023 at 04:43:35PM -0700, Luis Chamberlain wrote:
> > - }
> > - iterate_and_advance(i, bytes, base, len, off,
> > - copyin(p + off, base, len),
> > - memcpy_from_iter(i, p + off, base, len)
> > - )
> > - kunmap_atomic(kaddr);
> > - return bytes;
> > +
> > + do {
> > + char *p;
> > +
> > + n = bytes - copied;
> > + if (PageHighMem(page)) {
> > + page += offset / PAGE_SIZE;
>
> I don't quite follow here how before the page was not modified
> to get to the first kmap_atomic(page) and now immediately if we're
> on a PageHighMem(page) we're doing some arithmetic to the page
> address to get the first kmap_atomic(). The only thing I could
> think of is seems like an implicit assumption here that if its a compound
> highmem page then we always start off with offset with a value of
> 0, is that right? But that seems to not be correct either.
The important thing to know is that it was buggy before. If you called
copy_page_from_iter_atomic() with an offset larger than PAGE_SIZE, it
corrupted random memory! I can only assume that nobody was doing that.
